Add json type conversion

[RobinPicard]

Addresses #1722

This PR adds 2 class methods to the JsonSchema class to check whether an object is a JSON schema type and to convert such an object to a different JSON schema type

This change has 2 main objectives:

• Provide intercompatibility between all different supported JSON schema types such that the user could provide any of them even if the model they use support only one in the list. This can be useful for some API-based models (Gemini)
• Simplify the code in the format_output_type methods of the model type adapters. We want to push the complexity upstream such that the models only have to deal with the JsonSchema class instead of having to treat different JSON schema types itself

[rlouf]

Why not have different methods?

[RobinPicard]

I thought it would be a more convenient interface to have only one method through which one can list all the types they supports.

For instance:

if JsonSchema.is_json_schema(output_type):
    return JsonSchema.convert_to(
        output_type,
        ["dataclass", "typeddict", "pydantic"]
    )
)